Custom validators in Flutter - UI Render Trace

Component - Custom validators

This UI component shows a simple form with a text input field that uses a custom validator function. The validator checks if the input text meets specific rules before allowing submission.

Widget Tree

Scaffold
├─ AppBar
│  └─ Text
└─ Padding
   └─ Form
      └─ Column
         ├─ TextFormField
         └─ ElevatedButton

The Scaffold provides the basic page structure with an AppBar at the top showing a title. Inside the body, Padding adds space around the form. The Form widget holds a Column that stacks the TextFormField input and a submit ElevatedButton vertically.

Render Trace - 6 Steps

Step 1: Scaffold

Step 2: Padding

Step 3: Form

Step 4: Column

Step 5: TextFormField

Step 6: ElevatedButton

State Change - Re-render

Trigger:User taps the 'Submit' button

Before

Form input is empty or invalid

→

After

Validator runs, shows error message if input invalid, or allows submission if valid

Re-renders:TextFormField rebuilds to show error message if validation fails

UI Quiz - 3 Questions

What happens when the user taps the Submit button with invalid input?

A The app closes immediately

B The TextFormField shows an error message below the input

C Nothing happens, the button is disabled

D The input text is cleared automatically

Key Insight

Using custom validators inside a Form widget helps ensure user input meets your app's rules before proceeding. The validator function runs when the user tries to submit, and the UI updates to show helpful error messages if needed. This improves user experience by giving immediate feedback.